1. Field of the Invention
The present invention relates to a scanning probe microscope in which a force exerted between a cantilever provided on its top end with a minute probe and a sample is used for measuring an uneven shape and/or a physical characteristic of a surface of the sample and, particularly, to a scanning probe microscope in which contact between a sample and a probe can be prevented.
2. Background Art
A scanning probe microscope comprises a cantilever having a minute probe on a top end thereof, displacement detecting means for detecting displacement of the cantilever, Z-axis controlling amount calculating means for calculating a controlling amount for keeping constant a displacement amount of the cantilever or an oscillation amount in oscillating the cantilever, Z-axis driving means for driving in a Z direction the cantilever or a sample on the basis of the controlling amount from the Z-axis controlling amount calculating means, XY scanning means for relatively moving the probe in a direction of an XY plane with respect to the sample and storing means for storing the controlling amount outputted from the Z-axis controlling amount calculating means and a signal outputted from the displacement detecting means. In the scanning probe microscope, the probe scans a surface of the sample by means of the XY scanning means so as to measure an uneven shape and/or a physical characteristic of the surface of the sample. An atomic force microscope (AFM) for detecting atomic force such as Van der Waals force exerted between a top end of a probe and a surface of a sample is known as an example of the above.
The atomic force microscope includes Z-axis controlling means for controlling a distance between a probe and a sample so that displacement of the cantilever would be kept constant. The atomic force microscope is a scanning probe microscope in which the Z-axis controlling amount is converted into an image so as to convert an uneven shape of a surface of the sample into a three-dimensional image.
When an end of a small piece sample such as a semiconductor is measured in a scanning probe microscope, in the case that there is an area where no sample is in an area, the Z-axis driving means are lowered in the area with no sample to a lower limit position in a driving range for the purpose of detecting a nonexistent sample, so that the probe is vainly moved in the air for scanning. Scanning under such a condition is likely to cause breakage of the probe due to contact between the probe and the sample when a scanning speed is faster than a moving speed of the probe to a top surface of the sample after the probe detects the sample although the Z-axis driving means are retracted in order to raise the probe to the top surface of the sample when the probe detects a side surface of the sample.
Therefore, in order to avoid the contact between the probe and the sample, used are a method for scanning at a speed low enough for the probe to safely move to the top surface of the sample or a method for observing a height of the probe in scanning so as to stop scanning once to move the probe to a safe position before restarting scanning in the case of exceeding a previously set value of limit in height of the probe (refer to Patent Reference 1, for example).
The above-mentioned methods, however, have the following problems.
1) It takes excess time for the measurement, so that the throughput is lowered.
2) Scanning is stopped once in the case of using a piezo electric element or the like as a scanning device, so that a nonlinear characteristic of the piezo electric element causes distortion of an image.
[Patent Reference 1] JP-A-2003-166927 (Paragraphs 0025-0029, FIG. 4)